Total synthesis of marine and terrestrial natural products and development of novel photochromic ligands for ion channels, G-protein coupled receptors and cytoskeletal proteins

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der Fakultät für Chemie und Pharmazie

der Ludwig-Maximilians-Universität München

Total Synthesis of Marine and Terrestrial

Natural Products

And

Development of Novel Photochromic Ligands

for Ion Channels, G-Protein Coupled Receptors

and Cytoskeletal Proteins

Nils Sebastian Winter

aus München, Deutschland

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Erklärung

Diese Dissertation wurde im Sinne von § 7 der Promotionsordnung vom 28. November 2011 von Herrn Prof. Dr. Dirk Trauner betreut.

Eidesstattliche Versicherung

Diese Dissertation wurde eigenständig und ohne unerlaubte Hilfe erarbeitet.

München, den 30.01.2018

Nils Sebastian Winter

Dissertation eingereicht am: 01.02.2018

1. Gutachter: Prof. Dr. Dirk Trauner

2. Gutachter: Prof. Dr. Oliver Trapp

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Parts of this work have been published in peer-reviewed journals:

 “Optical Control of Dopamine Receptors Using a Photoswitchable Tethered Inverse Agonist” – Prashant C. Donthamsetti*_{, Nils Winter}*_{, Matthias}

Schönberger, Cherise Stanley, Jonathan A. Javitch, Ehud Y. Isacoff and Dirk Trauner, J. Am. Chem. Soc. 2017, 139, 18522–18535.

*equal contributions

 “Thiocarbonyl Ylide Chemistry Enables a Concise Synthesis of (±)-Hippolachnin A” – Nils Winter and Dirk Trauner, J. Am. Chem. Soc. 2017, 139, 11706−11709. Highlighted in Synfacts DOI: 10.1055/s-0036-1591428

Parts of this work have been presented on scientific conferences:

 “Total Synthesis of Hippolachnin A” – Winter, N., Trauner, D. International Society of Heterocyclic Chemistry 2017, Regensburg, Germany

Parts of this work are currently prepared for publication:

_{“Novel Photochromic Channel Blockers for the Two-Pore Domain Potassium}

Channel TREK-1” – Philipp Leippe,#_{Nils Winter,}#_{Martin P. Sumser Dirk}

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Abstract

Chapter I: Synthesis of the Marine Natural Product

Hippolachnin A

Marine organisms are a rich source of structurally diverse secondary metabolites.1-3_In

this regard, sponges have been intensively studied and have provided a large fraction of such natural products. In 2013, Lin and co-workers reported the isolation of the polyketide natural product hippolachnin A, together with its proposed biogenetic precursor, from the ethanolic extract of the South China Sea sponge Hippospongia Lachne collected from the waters at Xisha Islands.4_{Hippolachnin A exhibited potent}

antifungal activity against the pathogenic fungi Cryptococcus neoformans, Trichophyton rubrum, and Microsporum gypseum with an MIC value of 0.41 µM for each fungus. The following work describes the total synthesis of hippolachnin A. The bicyclo[3.2.0]hepatadienenone core is installed via a disrotatory 4π-electrocyclization followed by excited state rearrangement of -tropolone methyl ether. The synthetically challenging installation of the vicinal diethyl groups has been achieved by a [3+2] dipolar cycloaddition of a thiocarbonyl ylide to an electron-poor double bond. Chelation controlled trapping of an in situ generated carbocation by a tin enolate led to closure of the final tetrahydrofuran ring. The synthetic route provided over 100 mg of the natural product as well as access to derivatives thereof (Scheme A).

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Chapter II: Synthetic Studies Towards the Meroterpenoids

Bisacremin E, F and G.

Endophytic fungi grow within a plant host and usually live in a symbiotic relationship. Their growth involves continual metabolic interaction between fungus and host.5

Therefore endophytic fungi display a rich source of secondary metabolites often bearing interesting bioactivity.6_{In 2015, Wei and co-workers discovered bisacremines E–G from}

a culture extract of the soil-derived strain A. persicinum SC0105.7_{Bisacremine E and F}

are supposed to be derived from a formal [4+2] cycloaddition of two acremine F monomers, followed by dehydration to form the final tetrahydrofuran ring (scheme B). Though both the endo and the exo transition state would lead to a natural product, we assumed that this reaction would not require enzymatic catalysis but could proceed spontaneously in solution.

SCHEME B: Proposed biosynthesis of bisacremine E–G.7

This work describes the total synthesis of acremine F as well as studies towards its dimerization to form bisacremines E and F. Therefore, we investigated several mechanistic hypotheses for a biomimetic dimerization, including a cationic cascade, a radical cation Diels-Alder reaction and photochemical [2+2]-cycloaddition followed by formal Cope rearrangement to form the core of the natural products. Furthermore, studies towards the non-biomimetic synthesis of bisacremine G, involving a vinyl quinone Diels-Alder (VQDA) reaction, have been conducted (Scheme C).

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SCHEME C: Retrosynthetic analysis of bisacremine G.

Chapter III: Exerting Photocontrol over G-Protein Coupled

Receptors

Chapter three of this thesis describes the synthesis of light tunable modulators of G-protein coupled receptors (GPCRs). In detail, the synthesis of multiple tethered and non-tethered photoswitchable congeners derived from the known dopamine receptor (DAR) agonist PPHT are described as well as their ability to modulate DAR function in cellulo. Furthermore, the synthesis of light tunable derivatives of the known mAChR superagonist iperoxo is described as well as their effects when being applied to Langendorff preparations and cells transiently expressing mAChRs. The last part of this chapter describes the synthesis of photoswitchable derivatives of the TAS2R agonist denatonium (Figure A).

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Chapter IV: Development of Photochromic Ion Channel

Blockers

Chapter four of this thesis describes the synthesis of photoswitchable modulators of voltage gated ion channels and their evaluation in cellular systems. Therefore, we designed azobenzene containing derivatives of A-803467, a selective channel blocker of Nav1.8, bupivacaine, a local anesthetic used in intensive medicine and raxatrigine, an

anesthetic in clinical phase three. The ability of azo-bupivacain derivatives to photoblock voltage gated ion channels has been examined in HEK293T cells transiently overexpressing Kv2.1 and TREK channels as well as in mouse hippocampal neurons.

The effect of azo-raxatrigine has been evaluated using high-throughput methods on cells stably expressing Nav1.5, Nav1.7 and Kv1.3 (Figure B).

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Chapter V: Studies Towards the Light-Dependent Regulation of

Cytoskeletal Proteins

Chapter five of this thesis describes the development of photoswitchable modulators of the actin cytoskeleton network. In particular, a variety of light dependent inhibitors of Arp2/3 based on the selective inhibitors CK636 and CK666 have been synthesized and evaluated in in vitro assays confirming their ability to inhibit Arp2/3 dependent actin polymerization.

FIGURE C: Molecular structures of CK-666, CK-636 and blebbistatin.

Furthermore, we developed a robust route to azobenzene containing derivatives of the selective nonmuscular myosin II inhibitor blebbistatin which could be used for the investigation of myosin II dynamics in the actin network (Figure C).

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Acknowledgements

First and foremost, I would like to thank my supervisor Prof. Dr. Dirk Trauner for giving me the opportunity to conduct my PhD in his group and for his support in the course of my PhD thesis. I am more than grateful for the challenging projects he provided and the freedom I was granted while working on them. His enthusiasm and patience for chemistry and for science in general are highly inspiring. Under his guidance I became a better a chemist and human.

Furthermore, I am very thankful to Prof. Dr. Oliver Trapp for agreeing to be the second reviewer of this thesis. I would also like to thank Prof. Dr. Anja Hoffmann-Röder, Prof. Dr. Konstantin Karaghiosoff, Prof. Dr. Ramus Linser and Dr. Dorian Didier for being on my defense committee.

I want to thank Dr. Julius Reyes, Dr. Nina Hartrampf, Felix Hartrampf, Dr. Martin Sumser, Daniel Terwilliger and Benjamin Williams who spend a considerable amount of time and effort on proofreading parts of this manuscript.

Further, I want to acknowledge my talented and very motivated students Dennis Rhein, Lina Judkele, Franziska Schüppel, Desirée Heerdegen, Alexander Kremsmair, Brigitta Bachmair, Elena Reinhardt, Henriette Lämmermann and Ebru Durak who did an amazing work and helped me a lot.

I am very thankful to all members of the Trauner and the Magauer group who made working during my PhD such a pleasant and inspiring experience. I especially want to thank Dr. Hongdong Hao and Dr. Julius Reyes for their advice and their great friendship in good and in hard times.

I would also like to thank the permanent staff of the Trauner group, Carrie Louis, Aleksandra Grilic, Luis de la Osa de la Rosa and Heike Traub for keeping the group running and always offering a helping hand.

Additionally, I would like to thank all the great people from different groups I was allowed to collaborate with, from whom I really learned a lot and who led me grow as a scientist: Dr. Andrea Brüggemann, Dr. Prashant Donthamsetti, Prof. Dr. Ehud Isacoff, Prof. Dr. Annette Nicke, Prof. Dr. Philipp Sasse, Dr. Maik Behrens, Prof. Dr. Erwin Sigel, Abou Ghali Majdouline, Dr. Julie Plastino and Prof. Dr. Marc Stadler.

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I am also very grateful for the great work of the members of the analytical department: Dr. Werner Spahl, Sonja Kosak, Dr. David Stevenson, Claudia Dubler and Dr. Peter Mayer.

Most importantly, I want to thank my family for the great and sustained support they gave me and, especially Sarah for everything she has done. Without you all of this would not have been possible.

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List of abbreviations

2,2-DMP 2,2-dimethoxypropane

Å Ångström

ADP adenosine diphosphate ADHD attention deficient

hyperactivity disorder ADTN 2-amino-6,7-dihydroxy

tetrahydronaphthalene

Ac acetyl

ACh acetylcholine

ATP adenosine triphosphate ATR attenuated total reflection AIDS aaquired immune

deficiency syndrome Arp2/3 actin related protein

complex

BHT butylated hydroxytoluene Boc t-butyloxycarbonyl

° C degree Celsius

cAMP cyclic adenosine monophosphate CAN ceric ammonium nitrate CBS Corey-Bakshi-Shibata CDI 1-1'-Carbonyldiimidazole CNS central nervous system

CoA coenzyme A

CSA camphersulfonic acid

d dublet (NMR)

  chemical shift (NMR)

DA dopamine

DAG diacylglycerol DAR dopamine receptor DAMP dimethyl diazomethyl

phosphonate DCC dicyclohexylcarbodiimide DCB 1,4-dicyanobenzene DCE 1,2-dichloroethane DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone

DIBAL-H diisobutylaluminum hydride DIPA diisopropylamine

DIPEA diisopropylethylamine DMAP 4-(dimethylamino) pyridine DME 1,2-dimethoxyethane DMF dimethylforamide DMP dess-martin periodinane DMSO dimethylsulfoxide DRG dorsal root ganglion

E opposite (trans)

ECD electron capture detector

ee enantiomeric excess EI electron impact ionization ESI electrospray ionization

Et ethyl

EtOAc ethyl acetate

FVP flash vacuum pyrrolysis

G gramm

GABA -aminobutyric acid GDP guanosine diphosphate GIRK G-protein-coupled

inwardly-rectifying potasium channel GPCR G-protein coupled receptor GTP guanosine triphosphate

H hour

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HWE Horner Wadsworth Emmons

IC50 half maximal inhibitory

concentration IP3 inositol triphosphate

IR infrared

J coupling constant (NMR)

kDa kilodalton

Kv voltage gated potassium

channel

 wavelenght

L laevus (left)

LDA lithium diisopropylamide LED light-emitting diode LiHMDS lithium bis(trimethyl

silyl)amide

m medium (IR)

m meter

m multiplett (NMR)

M2 myosin 2

mAChR muscarinic acetylcholine receptor µM micromolar Me methyl MeCN acetonitrile Mes-Acr-Ph 9-mesityl-3,6-di-tert-butyl-10-phenylacridini um tetrafluoro borate MIC minimal inhibitory

concentration

min minutes

mmol milimole

MS mass spectroscopy

MSN spiny neuron

nAChR nicotinic acetylcholine receptor

Nav voltage gated sodium

channel NBS N-bromosuccinimide n-BuLi n-butyllithium NEt3 triethylamine NHS N-hydroxysuccinimide Nm nanometer NM II nonmuscular myosin II NMDA N-methyl–D-aspartate NMR nuclear magnetic resonance NPF nucleaction proliferation factor oDCB 1,2-dichlorobenzene PBS phosphate-buffered saline PCC pyridinium chlorochromate PDE phosphodiesterase PEG polyethylene glycol

Ph phenyl PhMe toluene PIFA (bis(trifluoroacetoxy) iodo)benzene PIP2 phosphatidylinositol-4.5-bisphospahte PLC phospholipase C PMB para-methoxybenzyl

PNS peripheral nervous system Ppm parts per million

PPTS pyridinium p-toluenesulfonate PTSA p-toluenesulfonic acid

q quartet (NMR)

R undefined substituent Rf retardation factor

s strong (IR)

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t time

T temperature

TAS2R taste receptor TBAF tetrabutylammonium fluoride TBTA tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine TEMPO 2,2,6,6-tetramethyl piperinyloxyl THF tetrahydrofuran

TMANO trimethylamine N-oxide TBS tert-butyldimethylsilyl

TM transmembrane domaine

TMS trimethylsilyl TIPS triisopropylsilyl TFA trifluoroacetic acid TFE trifluoroethanol UV ultra violet

VQDA vinyl quinone diels alder

w weak (IR)

wt% weight percent

WASP Wiskott-Aldrich syndrome family protein

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Chapter I

Synthesis of the Marine Natural

Product Hippolachnin A

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1.1 Introduction

1.1.1 Plakortin and Gracilioether natural products

Marine organisms are a rich source of structurally diverse secondary metabolites.1-3_In

this regard sponges have been largely studied and have provided a large fraction of such natural products. The plakortin family is a large class of polyketide natural products possessing high antimicrobial and antifungal activity derived from sponges of the genus Plakortis (Figure 1.1). Notably, many members of this family contain a cyclic peroxide.2

Faulkner reported the first isolation of plakortin (1.1), a six-membered cycloperoxide, from P. Halicondrioides in 1978.8_{However, it was not until 1999 that the absolute}

stereochemistry was assigned by Fattorusso.9_{Subsequently, a series of related}

bioactive molecules have been isolated wherein the side chain at C6 differs by the length of the carbon chain demonstrating the diversity of building units that can be incorporated by the polyketide synthase.3, 10_{While most of the isolated natural products offer some}

bioactivity, the plakortone-series proved to be highly potent activators of cardiac SR-Ca2+

ATPase.11

FIGURE 1.1: A selection of plakortin natural products.

In 2009, in hope of identifying potent leads from marine invertebrates against malaria infections, Fusetani and co-workers isolated the first members of the gracilioether family, a sub-class of the plakortin natural products, gracilioether A-C.12_{While all three isolated}

compounds showed antimalarial activity, only gracilioether A (1.8) featured a structurally interesting heterotricyclic ring system containing a six-membered endoperoxide (Figure

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1.2). In 2012 the group of Zampella found further members of the gracilioethers, namely gracilioether E-J,13_{and in 2013 the further oxidized natural product gracilioether K (1.9).}14

While gracilioether E-G, I and J were almost completely inactive against all screened targets, gracilioether H (1.10) showed antimalarial activity against a chloroquine-resistant strain, proving the importance of the endoperoxide for bioactivity.13

FIGURE 1.2: Members of the gracilioether family.

1.1.2 Synthetic approaches towards the gracilioether family

Due to their impressive structures and the diverse bioactivities, synthetic chemists immediately pursued de novo syntheses of members of this family. In 2014, Brown and co-workers reported the first synthesis of gracilioether F (1.14) (Scheme 1.1),15_relying

on a late-stage C-H functionalization. Ketene formation from acid chloride 1.18 and subsequent [2+2]-cycloaddition with cyclopentene 1.19 furnished bicyclo[3.2.0]heptane

1.20, which after alkylation and Baeyer-Villiger oxidation gave rise to bicyclic lactone 1.21. After cleavage of the double bond and oxidative work-up, carboxylic acid 1.22 was

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SCHEME 1.1: Brown’s synthesis of gracilioether F (1.14).15

The first asymmetric total synthesis of gracilioether B (1.11) and C (1.12) was accomplished by Sorenson and co-workers in 2015 (Scheme 1.2).16_{Alkylation of}

oxazolidinone 1.23 with allyl iodide 1.24 sets the C8 stereocenter providing 1.25 in good

yield. After dihydroxylation under Sharpless conditions and subsequent transesterification, lactone 1.26 was obtained, which could be converted into acetal 1.27 following reduction with DIBAL-H. Aldehyde 1.28 was then generated by Ley oxidation. Weiler alkylation then accessed -ketoester 1.29, which directly underwent transacetalization to form hemiacetal 1.30 and dehydration in the same step to furnish furanylide 1.31. Transacetalization of 1.31 catalyzed by In(OTf)3, followed by immediate

Horner−Wadsworth−Emmons olefination with diethyl (2-oxopropyl)phosphonate afforded gracilioether B (1.11). Stereoselective reduction using Ru-catalysis then afforded gracilioether C (1.12) in good yield.

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SCHEME 1.2: Sorenson’s synthesis of gracilioether B (1.11) and C (1.12).16

Carreira and co-workers reported the first total synthesis of gracilioether E (1.13) in 2016.17_{The tetrahydrofuran ring was obtained by a Lewis-acid catalyzed ene-cyclization}

of enone 1.33. Oxidative cleavage of the double bond then provided cyclobutanone 1.34, which was then subjected to Baeyer-Villiger oxidation to obtain the full heterocyclic core of 1.35. Installation of the (Z)-configured double bond by selenoxide elimination then furnished the natural product 1.13 (Scheme 1.3).

SCHEME 1.3: Carreira’s synthesis of gracilioether E (1.13).17

Carreira applied a similar strategy to access gracilioether F (1.14), a congener lacking the vinylogous carbonate moiety. Prins-type cyclization of tertiary alcohol 1.37 with paraformaldehyde gave rise to tricycle 1.38 as a 5:1 mixture of diastereoisomers in moderate yield. Oxidative cleavage followed by Bayer-Villiger oxidation accessed the full core of 1.39. The natural product 1.14 was then obtained after oxidation of the most electron-rich C-H bond using CrO3 (Scheme 1.4).

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SCHEME 1.4: Carreira’s synthesis of gracilioether F (1.14).17

A biosynthetically-inspired total synthesis of gracilioether F (1.14) was reported by Wong and co-workers in 2016.18_{Though the plakortin family of natural products contains many}

cyclic peroxides, he proposed that a 1,2-dioxane similar to 1.41 could serve as a common precursor to gracilioether members in a plausible biomimetic manner. Reductive cleavage of the O-O bond followed by oxidation could then for example access 1.14. The synthesis commences with allene 1.42 which was transformed into lactone 1.43 in three steps. Further manipulations then gave rise to vinyl iodide 1.44. Heck cyclization then provided diene 1.45 which underwent a Diels-Alder cycloaddition with singlet oxygen to obtain unstable dihydrodioxine 1.46. 1.46 was immediately reduced to the stable 1,2-dioxane 1.41 utilizing diimide generated from dipotassium azodicarboxylate. Gracilioether F (1.14) was then furnished following

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SCHEME 1.5: Wong’s synthesis of gracilioether F (1.14).18 reductive cleavage of the O-O bond and twofold oxidation (Scheme 1.5).

In 2017, Wu and co-workers reported the syntheses of a variety of gracilioethers based on common precursor 1.49 (Scheme 1.6).19_{Palladium-catalyzed hydrocarbonylation of}

alkyne 1.50 provided rapid access to lactone 1.51. Further manipulations by alkylation, hydroboration and oxidation provided plakilactone C (1.52) which could then undergo a Sm(II)-mediated Michael-addition to yield ketone 1.53. Silyl enol ether formation followed by oxidative cleavage furnished aldehyde 1.49 as a divergent precursor. Grignard addition and C-H oxidation accessed the proposed structure of gracilioether I (1.16). The C-H oxidation proved a challenging problem. All methods for this type of transformation failed, presumably due to the adjacent carbonyl group. Nevertheless, a mixture of Pb(OAc)4 and iodine accomplished the transformation. Unexpectedly, the 1H and 13C

NMR data were not consistent with those reported by Zampella in 2012, suggesting that

1.16 had been incorrectly assigned.

SCHEME 1.6: Wu’s synthesis of the proposed structure of gracilioether I (1.16).19

Starting from alcohol 1.54, Wu was also able to complete the formal synthesis of gracilioether F (1.14) (Scheme 1.7). Without the carbonyl group at C1, the C-H oxidation

proceeded well under standard conditions, giving cyclic ether 1.55. Attempts to oxidize the C1 position in order to obtain 1.16 resulted in oxidative cleavage of the C-C bond at

position C8, providing lactone 1.56. The synthesis of 1.14 was then accomplished as

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SCHEME 1.7: Wu’s formal synthesis of gracilioether F (1.14).19

Wu’s entry to gracilioether E (1.13) began with DIBAL-H reduction of aldehyde 1.49 followed by the addition of TiCl4 and Et3SiH to afford, unexpectedly, acetal 1.57.

Mukaiyama aldol addition and C-H oxidation, as previously described, accessed 1.58. Oxidation of the most electron-rich C-H bond gave lactone 1.59, which could then be converted into gracilioether E (1.13) as described by Carreira17_{(Scheme 1.8).}

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1.1.3 Hippolachnin A – Isolation and Biosynthesis

In 2013, Lin and co-workers reported the isolation of the polyketide natural product hippolachnin A (1.61), together with its proposed biogenetic precursor 1.62, from the ethanolic extract of the South China Sea sponge Hippospongia Lachne collected from the Xisha Islands (Figure 1.3).4_{While 1.62 was already disclosed as a PPAR}

antagonist,21_{hippolachnin A (1.61) exhibited potent antifungal activity against the}

pathogenic fungi Cryptococcus neoformans, Trichophyton rubrum, and Microsporum gypseum with an MIC value of 0.41 µM for each fungus. The structure 1.61 was elucidated based on NMR, IR and mass spectral data, comparison of the calculated and measured ECD spectra confirmed the absolute stereochemistry of the molecule. Hippolachnin A consists of a heterotricyclic core featuring six contiguous stereocenters and bears an unusual array of four ethyl groups on its convex face. It also contains a (Z)-configured vinylogous carbonate.

FIGURE 1.3: Hippolachnin A (1.61) and its proposed biogenetic precursor 1.62.

Lin’s biosynthetic proposal of hippolachnin A is shown in Scheme 1.9.4_{The ethyl groups}

of the natural product are proposed to be derived from butyrate units (1.63). Kubanek and Anderson furthermore, proposed that the activated butyrate arises from a reduction-dehydration-reduction sequence of -ketoester 1.64.22_{1.64 itself is derived from the}

condensation of malonyl CoA (1.65) with acetyl CoA (1.66) followed by decarboxylation. Four butyrate and one acetate units are then assembled by a polyketide synthase to form tetraene 1.67. Oxidation to epoxide 1.68 followed by cyclization then furnishes 2,3-dihydrofuran 1.69, which upon elimination and transesterification gives cyclization precursor 1.70. 1.70 subsequently undergoes a

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SCHEME 1.9: Proposed biosynthetic formation of hippolachnin A (1.61).4

[2+2]-cycloaddition to form the natural product.

1.1.4 Hippolachnin A as a target in total synthesis

Although hippolachnin A is proposed to be derived from lactone 1.62, the latter had already been isolated in 1980 from Plakortis halichondrioides.23_{In 2005, Ohira and}

co-workers reported the first total synthesis of 1.62, relying on a late-stage C-H insertion.24

Diol 1.71 was prepared from -caprolactone (1.72) by a four-step sequence. Further manipulations gave rise to the key intermediate 1.73. Treatment of 1.73 with dimethyl diazomethylphosphonate (DAMP) first generated the vinylidene carbene from the aldehyde, which could undergo a 1,2-hydride shift to give alkyne 1.74. A second equivalent of DAMP then generated the vinylidene carbene from the ketone that further underwent a 1,5 C-H insertion, forming dihydrofuran 1.75. Alkene 1.76 was then obtained by a short alkylation and reduction sequence. Further manipulation accessed methyl ester 1.77 as an inconsequential mixture of diastereoisomers which could be transformed into the natural product 1.62 (Scheme 1.10).

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SCHEME 1.10: Ohira’s synthesis of lactone 1.62.24

The first total synthesis of hippolachnin A (1.61) was reported in 2014 by Carreira and co-workers (Scheme 1.11).20_{His strategy relies on a [2+2]-cycloaddition to access the}

bicyclo[3.2.0]heptane core and a Lewis acid catalyzed ene-reaction to forge the tetrahydrofuran ring. The synthesis begins with a photochemical [2+2]-cycloaddition of cyclopentenone 1.80 with 3-hexyne (1.81), furnishing bicyclo[3.2.0]heptene 1.82. Subsequent elimination under mildly basic conditions provided enone 1.83. Stereoselective cuprate addition gave rise to ketone 1.84 that was then alkylated under Knochel’s conditions to give tertiary alcohol 1.85. While it proved difficult to alkylate 1.85, vinylogous carbonate 1.86 was obtained when 1.85 was stirred neat in (E)-methyl-3-methoxyacrylate (1.87) in the presence of PPTS for prolonged reaction times. Lewis acid-catalyzed ene-cyclization accessed key cyclobutane 1.88 that could selectively be reduced under heterogeneous hydrogenation conditions to give the

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kinetic product 1.89. Further -phenylselenation followed by oxidation and elimination allowed for the clean formation of hippolachnin A (1.61).

Shortly after Carreira published his synthesis, Gosh reported his studies towards the asymmetric total synthesis of 1.61 employing a ring-closing metathesis and a photochemical [2+2]-cycloaddition (Scheme 1.12).25_{Ethyl ester 1.90 was derived from}

D-mannitol and could be transformed by a three-step procedure into aldehyde 1.91. After further manipulations, key triene 1.92 was obtained. Ring-closing metathesis afforded 2,3-dihydrofuran 1.93a and 1.93b as a 1:1 mixture of diastereoisomers at C7. 1.93b was

then transformed into the cycloaddition precursor 1.94 by a four-step protocol. Photochemical [2+2]-cycloaddition followed by hydrogenation of the vinyl group then provided lactone 1.95, which is only lacking the vinylogous carbonate moiety and one ethyl group of the natural product. However, using D-mannitol as the starting material would lead to the unnatural enantiomer of hippolachnin A (1.61).

SCHEME 1.12: Gosh’s synthetic studies towards hippolachnin A (1.61).25

In 2016, Brown and Wood reported a collaborative synthesis of hippolachnin A (1.61),26

the shortest up to date. The cyclobutane ring was formed through a [2+2+2]-cycloaddition of quadricyclane (1.95) to acid chloride 1.96 and the heterocycle by a late-stage allylic C−H oxidation (Scheme 1.13). Tricycle 1.97 underwent efficient ring-opening metathesis employing ethylene to form diene 1.98. Allylic C-H oxidation using White’s catalyst then provided lactone 1.99 in good yield. Aldol condensation followed by reduction of both double bonds furnished vinylogous carbonate 1.100 as a mixture of (Z)- and (E)- isomers. Finally, transesterification yielded the natural product 1.61.

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SCHEME 1.13: Wood and Brown’s collaborative synthesis of hippolachnin A (1.61).26 A biomimetic strategy towards 1.61 was reported in early 2017 by Wu (Scheme 1.14).19

1.101 was obtained by employing conditions previously reported by Ohira with slight

modifications. Even though hippolachnin A (1.61) is proposed to be derived from lactone

1.62, irradiation of 1.62 with a mercury lamp did not trigger the desired

[2+2]-cycloaddition but only led to isomerization of the double bond of the side chain. While the [2+2]-cycloaddition using the conjugate diene did not proceed, the Cu-catalyzed reaction of the isolated double bond gave access to Carreira’s intermediate 1.89 in low yield.

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A more efficient reaction was found however, when lactone 1.102 was irradiated in the presence of benzophenone as a photosensitizer (Scheme 1.15). Resultant cyclobutane

1.99, an advanced intermediate of Brown and Wood’s synthesis was obtained in good

yield, thus accomplishing a formal synthesis of hippolachnin A 1.61.

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1.2 Project outline

Opportunistic infections by ubiquitous fungi represent a major challenge to the immunocompromised. The yeast Cryptococcus neoformans, for instance, can cause life-threatening meningitis and affect the lungs and skin of patients with advanced acquired immunodeficiency syndrome (AIDS).27-30_{Hippolachnin A (1.61) was recently isolated}

from the South China Sea sponge Hippospongia lachne and proved to be highly potent against several pathogenic fungi, including C. neoformans (MIC = 0.41 μM).4_Therefore,

hippolachnin A (1.61) could provide an important lead in the search for novel drugs against these fungi.

Hippolachnin A (1.61) represents a unique synthetic challenge by virtue of bioactivity and molecular scaffold. At the beginning of the project, no complete synthesis of this natural product had been reported. Above all, the caged 4-5-5 heterotricyclic core featuring six contiguous stereocenters was expected to cause several difficulties in its preparation. Furthermore, the thermodynamically less favoured (Z)-double bond provides an additional distinct challenge. The goal of this project was to provide a scalable synthetic route also enabling the synthesis derivatives of the natural product and probe their bioactivity.

Therefore, we identified two possible retrosynthetic analyses of 1.61. In the first retrosynthesis, we envisioned installation of the vinylogous carbonate late in the synthesis from lactone 1.99. The cyclobutane and the tetrahydrofuran would simultaneously be formed by an intramolecular [2+2]-cycloaddition of a precursor containing a cyclopentene scaffold linked to an alkene (Figure 1.4).

FIGURE 1.4: First retrosynthetic analysis of 1.61.

Alternatively, the tetrahydrofuran ring of the natural product would be forged by a chelation-controlled trapping of a -ketoester enolate by an in situ generated carbocation,31_{forming the kinetic product (Figure 1.5). The vicinal diethyl groups would}

be accessed by a three-component coupling using ,-unsaturated ester 1.104 and an ethyl nucleophile and ethyl electrophile. 1.104 should be accessible via

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bicyclo[2.1.0]heptadienone 1.105 which would be formed from tropolone methyl ether

1.106 by disrotatory 4-electrocyclization followed by excited state rearrangement.32

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1.3 Results and Discussion

1.3.1 First generation approach – Retrosynthetic analysis

The first retrosynthetic analysis of hippolachnin A (1.61) is depicted in Scheme 1.16. It was envisioned to access 1.61 through Peterson olefination of lactone 1.99, which would be derived by an intramolecular [2+2]-cycloaddition following esterification of acid 1.108 and tertiary alcohol 1.109. 1.108 would be obtained by hydroalumination of 3-hexyne (1.110) followed by trapping with CO2.33 Tertiary alcohol 1.109 would be provided by

retro-[4+2]-cycloaddition of bicycle 1.111,34_{derived from enone 1.112 by conjugate}

addition35_{followed by 1,2-addition.}36_{Additions to 1.112 should be governed by the shape}

of the bicycle and arrive from the convex side of the molecule. Schenk-ene reaction of dicyclopentadiene (1.113) with singlet oxygen would provide enone 1.112.

Scheme 1.16: Retrosynthetic analysis of hippolachnin A (1.61).

Even though the [2+2]-cycloaddition of an acyclic ,-unsaturated ester was anticipated to provide a major challenge, we envisioned that, conducting the reaction in an intramolecular fashion, might allow efficient trapping of the initial diradical.

1.3.2 The [2+2]-Cycloaddition approach towards hippolachnin A

The synthesis commenced with the preparation of tertiary alcohol 1.109 from dicyclopentadiene (1.113) (Scheme 1.17). Conjugate addition from the convex side of enone 1.112 obtained from the Alder-ene reaction of 1.113 with singlet oxygen afforded ketone 1.114. Subsequent 1,2-addition of EtMgBr mediated by LaCl3.2LiCl yielded

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bicycle by heat followed by in situ-trapping of the generated cyclopentadiene resulted only in recovered starting material or decomposition, flash vacuum pyrolysis (FVP) cleanly provided tertiary alcohol 1.109 in good yield. It is worth noting that a FVP-apparatus could be improvised, consisting of two vigreux columns connected to a cooled S-shaped tube.

SCHEME 1.17: Synthesis of coupling partners 1.109 and 1.108.

Hydroalumination of 3-hexyne (1.110) followed by aluminate formation allowed efficient trapping of carbon dioxide to carboxylic acid 1.108.

With both building blocks in hand, several conditions for the esterification have been screened (Table 1.1).

TABLE 1.1: Conditions of the esterification of alcohol (1.109) and acid (1.108).

R = Base Additive Temp. Observation

1 Cl (1.115) pyridine DMAP 0 °C - RT starting

material

2 Cl (1.115) i-PrMgCl none 0 °C starting

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3 Cl (1.115) n-BuLi AgSbF6 –78 °C - 0 °C decomposition

4

(1.116)

i-PrMgCl none 0 °C starting

material

5

(1.117)

pyridine DMAP RT - 100 °C starting material

6

(1.118)

none DMAP RT formation of

1.117 7 (1.119) NEt3 DMAP 40 °C formation of 1.117 8 (1.120) NEt3 none 40 °C formation of 1.117 9 (1.121) NEt3 DMAP RT formation of 1.117 10 (1.122)

n-BuLi none −50 °C - RT decomposition

Standard esterification conditions utilizing the acid chloride only resulted in isolation of the free acid after work up. Attempts to generate the naked acylium ion only led to decomposition of the starting materials. Using various mixed anhydrides as an electrophile provided anhydride 1.117 instead of the ester (Entry 6-9). 1.117 was found to be stable to column chromatography and proved to be inefficient in ester formation, even at elevated temperatures. While both the acid chloride and the mixed anhydride were able to react sufficiently with t-BuOK to form the t-butyl ester, acetylation of tertiary alcohol 1.109 with acetic anhydride and DMAP failed to show any traces of product even after prolonged reaction times.

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As the steric hindrance of the tertiary alcohol could not be overcome, we decided to install the fourth ethyl group at a later state of the synthesis. The revised retrosynthesis is depicted in Scheme 1.18.

FIGURE 1.18: Revised retrosynthetic analysis of 1.61.

We envisioned closing the tetrahydrofuran ring by hemiketal-formation of a -ketoester and a tertiary alcohol followed by dehydration. The tertiary alcohol would be generated by a Grignard addition to a ketone. The -ketoester should be derived by Claisen condensation of methyl acetate and lactone 1.124. Lactone 1.124 would be traced back to an intramolecular [2+2]-cycloaddition of an ester formed by alcohol 1.125 and acid

1.108. Allylic alcohol 1.125 would again be derived from cyclopentadiene (1.127).

Epoxidation of cyclopentadiene (1.127) followed by SN2' displacement with

ethylcyanocuprate provided allylic alcohol 1.125 in moderate yield (Scheme 1.19). Quantitative formation of the lithium alkoxide and subsequent quenching with acid chloride 1.115 gave rise to allylic ester 1.128.

SCHEME 1.19: Synthesis of allylic ester 1.128.

With the desired precursor in hand, we went on to screen the intramolecular [2+2]-cycloaddition of diene 1.128 (Table 1.2).

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TABLE 1.2: Screening of the intramolecular [2+2]-cycloaddition of 1.128.

Entry Solution Additive Observation

1 acetone starting material + (E) conformer

2 acetone / acetonitrile 1:1 starting material + (E) conformer 3 acetone / benzene 1:1 starting material + (E) conformer 4 acetone / CH2Cl2 1:1 starting material + (E) conformer

5 benzene benzophenone starting material + (E) conformer 6 methylene chloride benzophenone starting material + (E) conformer 7 acetonitrile benzophenone starting material + (E) conformer 8 acetone benzophenone starting material + (E) conformer

CH2Cl2 EtAlCl2 starting material

CH2Cl2 TiCl4 starting material

CH2Cl2 ZnBr2 starting material

CH2Cl2 Tf2NH starting material

All screening conditions were performed under high dilution conditions at 0.01 M, and

irradiation was achieved using 310 nm LEDs in a Rayonet®_{-reactor. Unfortunately, no}

cycloaddition product could be observed, and only starting material commingled with its (E)-configured isomer was isolated. The addition of Lewis acids to these types of systems is known to activate the carbonyl group of the ester, which could lead to a Prins-type cyclization. Unfortunately, only starting material was observed.

While excitation of the system proceeded smoothly as proved by the observation of the (E)-configured isomer, we wondered whether π-bond rotation was too fast to allow for capture of the diradical by the tethered olefin or whether the linker between the double bonds was too short, making it impossible for the double bonds to reach it other. In order to prevent isomerization, we wanted to incorporate the enone double bond into a ring system. This was achieved by stitching the ends of the ethyl groups together using a sulphur bridge. In order to increase the length between the two double bonds and allow the molecule to be more flexible, the ester moiety was elongated to a -ketoester (Scheme1.19).

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SCHEME 1.19: Synthesis of cyclization precursor 1.134.

Ring expansion of tetrahydrothiopyranone 1.129 using ethyl diazoacetate gave rise to thiepanone 1.130 which, after reduction, mesylation and elimination provided ethyl ester

1.132.37_{Saponification followed by activation of the carboxylic acid as the acid chloride}

and subsequent esterification accessed enoate 1.134 in moderate yields. With the cyclic cyclization precursor in hand, we investigated the [2+2]-cycloaddition (Table 1.3).

TABLE 1.3: Screening for the [2+2] cycloaddition under 310 nm irradiation.

Entry Solvent Observation

1 MeCN decomposition 2 CH2Cl2 decomposition

3 benzene decomposition 4 acetone decomposition

Unfortunately, illumination of 1.134 in different solvents showed no trace of product. After irradiation the clear solution became cloudy, and NMR analysis of the mixture indicated decomposition of the starting material. While excitation of the molecule was possible, no productive pathways could be observed, which might be due to a preferred conformation of the molecule where both olefins are pointing into opposite directions. In order to provide more flexibility to the molecule, -ketoester 1.136 was prepared (Scheme 1.20).

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SCHEME 1.20: Preparation of -ketoester 1.136.

Activation of acid 1.108 with CDI and subsequent Claisen condensation with t-BuOAc followed by ketalization gave dioxanone 1.137. Retro-[4+2]-cycloaddition and trapping of the resulting ketene with alcohol 1.125 resulted in formation of -ketoester 1.136.

TABLE 1.4: Screening of the [2+2]-cycloaddition of 1.136 under 310 nm irradiation.

Entry Solvent Observation

1 MeCN Complex mixture

2 CH2Cl2 Complex mixture

3 benzene Complex mixture

4 acetone Complex mixture

Irradiation of 1.136 using 310 nm LEDs resulted in a complex mixture (Table 1.4) independent of the solvent used. As a result of the unsuccessful formation of the bicyclo[3.2.0]heptane core, we decided to modify the strategy as described in the next chapter.

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1.3.3 Second generation synthesis – The tropolone route to

hippolachnin A

Reproduced with permission from Nils Winter and Dirk Trauner, J. Am. Chem. Soc.

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1.3.4 Synthesis of synthetic derivatives of hippolachnin A

Hippolachnin A was reported to offer interesting antifungal activity. In order to investigate structure-affinity relationships of hippolachnin A (1.61) and to improve the bioavailability of this barely water soluble molecule, we prepared several derivatives of the natural product.

To determine whether the (Z)-configured vinylogous carbonate was necessary for activity, derivatives lacking this group or having a different geometry were prepared. To improve the solubility in water, the thioether was oxidized to the sulfoxide and the sulfone. Scheme 1.21 shows the syntheses of the derivatives.

SCHEME 1.21: Syntheses of synthetic derivatives of 1.61.

These compounds, together with intermediates 1.99, 1.139 and the synthetic natural product were sent to Prof. Dr. Marc Stadler at the Helmholtz Centre for Infection Research in Braunschweig. Unfortunately, none of these compounds, including the natural product, showed any antifungal activity.

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1.4 Conclusion

In conclusion, a concise, scalable and modular synthesis of the complex polyketidal natural product hippolachnin A was achieved, and several derivatives thereof have been prepared. These successful studies supported biological investigations of hippolachnin A.

Initial attempts to close the tetrahydrofuran ring as well as the cyclobutane ring in a single step by a non-biomimetic [2+2]-cycloaddition failed with multiple precursors. Revising the approach and constructing the bicyclo[3.2.0]heptanone core via a 4-electrocyclization followed by exited state rearrangement provided rapid access to the core of hippolachnin A. While conventional methods to install the geminal diethyl groups suffered from low diastereoselectivity, the simultaneously installation of the two ethyl groups through 1,3-dipolar cycloaddition elegantly solved this problem. In order to close the eastern tetrahydrofuran ring, ester elongation was required. Attempted nucleophilic additions to the ester failed due to the remarkable steric congestion around this carbonyl group. This problem could be solved by converting the ester into the smaller and more reactive aldehyde. Chelation-controlled trapping of an in situ generated carbocation by a tin enolate of the -ketoester constructed this last ring. Final reductive desulfurization yielded hippolachnin A in good yield. Notably, more than 100 mg of hippolachnin A was produced in a single pass of the entire synthetic route.

It was reported that hippolachnin A provides interesting antifungal activity and therefore derivatives of the natural product have been prepared in order to figure out structure-affinity relationships and to increase the bioavailability of the molecule. The thioether, installed in the carbonyl ylide cycloaddition, provided a functional handle for further transformations which would have been difficult otherwise, considering that the natural product is lacking functional groups. Unfortunately, hippolachnin A, as well as all the synthetic derivatives proved to be inactive against the proposed target.

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Chapter II

Synthetic Studies Towards the

Meroterpenoids Bisacremin E, F and

G

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2.2 Introduction

2.1.1 Acremine natural products – Isolation and biosynthesis

Endophytic fungi grow within a plant host and usually live in a symbiotic relationship. Their growth involves continuous metabolic interaction between fungus and host.5

Therefore endophytic fungi display a rich source of secondary metabolites often bearing interesting bioactivity.6_{The genus Acremonium is part of this family and many bioactive}

natural products, including the -lactam antibiotic cephalosporin, the immunosuppresants cyclosporins, the tremorgenic indole-diterpenoids lolitrems and some prenylated phenol inhibitors of N-SMase have been isolated from this genus.39_In

2005, Torta and co-workers reported the isolation of six meroterpenoid natural products, acremine A-F (figure 2.1) from A20, a strain of Acreonium bissoides, isolated from grapevine leaves artificially inoculated with Plasmopora viticola.40_{The structure analysis}

was based on NMR and mass data. Furthermore, an X-ray crystal structure of acremine A confirmed the relative stereochemistry. The absolute stereochemistry was determined using Mosher’s method. Acremines A-D showed moderate Inhibition of P. viticola sporangia germination. All acremines possess either a cyclohexene or aromatic portion, which is likely derived via a polyketide pathway, and a prenyl unit. Further cyclization can then lead to either the benzofurane or the chromanone ring system.

FIGURE 2.1: Molecular structures of acremine A-F.

In 2008, Malpezzi and co-workers reported the isolation and structural assignment of acremine G (2.7),6_{the first dimeric member of the acremine family. It is believed that}

acremine G arises from a Diels-Alder reaction of prenyl benzoquinone 2.8, possibly derived from acremine B (2.2), and aromatic diene 2.9. Further oxidation could then furnish the natural product (scheme 2.1). Although this hypothesis makes sense from a

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synthetic point of view, neither of the possible Diels-Alder partners have been isolated to date.

SCHEME 2.1: Possible biosynthesis of acremine G (2.7).6

Five further members, acremines H-N (figure 2.2), have been isolated from the same strain in 2008 by Nasini and co-workers.39_{Acremine H and I possess one and two}

oxirane rings respectively. Acremine L and M can be derived from these by epoxide opening and further reduction or cyclization.

FIGURE 2.2: Molecular structures of acremine H-N.39

In 2013, the acremine family became even more diverse due to the isolation of chlorinated derivatives 5-chloroacremine A (2.16) and 5-chloroacremine H (2.17) together with acremine O (2.18), P (2.19), Q (2.20) and R (2.21) by Garson and co-workers.41_{In 2017, Garson revised the structure of acremine P (2.21).}42_{A further}

expansion of the acremine family was reported in 2014 when Afiyatullov and co-workers reported the isolation of acremine S (2.22) from the marine-derived fungus Isaria felina KMM 463943_{(Figure 2.3).}

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FIGURE 2.3: Molecular structure of 5-chloroacremine A and H and acremine O-R.39, 41-43

Until 2015, acremine G remained the only dimeric acremine natural product. When Wei and co-workers were analyzing a culture extract of the soil-derived strain A. persicinum SC0105 that showed activity against Staphylococcus aureus, they discovered bisacremines A-D together with their possible biogenic precursor acremine T (2.23).44

Bisacremines A-D are hypothesized to be derived from acremine T (2.23) and an oxirane precursor (2.24). 2.23 is assumed to nucleophilicaly open the epoxide of 2.24, generating an allylic cation that can then be intercepted by the generated tertiary alcohol to form bisacremines A-D. This hypothesis seems to be substantial, considering that all possible stereoisomers are natural products (Scheme 2.2).

SCHEME 2.2: Proposed biosynthesis of bisacremines A-D.44

Later in 2015, Wei discovered three more bisacremines E-G from the same extract.7

Bisacremine E (2.25) and F (2.26) are supposed to be derived by a formal [4+2] cycloaddition of two acremine F monomers, followed by dehydration to form the final tetrahydrofuran ring (scheme 2.3). As both the endo- and the exo-transition state of the Diels-Alder reaction lead to different isolated natural products, it might be plausible that this reaction is not catalyzed by an enzyme. Oxidation and intramolecular acetalization of bisacremine F, followed by aromatizative dehydration would then

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SCHEME 2.3: Proposed biosynthesis of bisacremine E-G.7

furnish bisacremine G (2.27). While bisacremine E-G were not cytotoxic against A549, MCF-7 and HepG2, bisacremine G showed dose–dependent effects in in vitro anti-inflammation assays.

2.1.2 Synthetic approaches

The first total synthesis of acremine A (2.1) was reported in 2010 by Mehta45_{and marks}

the first total synthesis of an acremine natural product in general. The synthesis starts with bicycle 2.30, which was converted into oxirane 2.31 through epoxidation followed by alkylation and reduction from the convex side of the system. Retro Diels-Alder reaction followed by oxidation and Wittig olefination gave rise to ,-unsaturated

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ester 2.32 in good yields. Diol 2.33 was then accessed by protection of the enone moiety and Grignard addition into the ester. Oxidation and reductive epoxide opening yielded ketone 2.34. Reduction of 2.34 using Luche’s conditions and further deprotection of the enone accessed acremine A (2.1) together with its epimer 2.35 in a 5:1 dr (Scheme 2.4).

From intermediate 2.33 Mehta was also able to access acremine I (2.12). Directed epoxidation and oxidation of the secondary alcohol gave enone 2.36. In this case, the Luche reduction had no preference for any side and provided secondary alcohol 2.37 as a 1:1 mixture of diastereoisomers. Hydrolysis of the ketal gave then rise to acremine I (2.12) and its stereoisomer 2.38 as a separable mixture (Scheme 2.5).

SCHEME 2.5: Mehta’s synthesis of acremine I (2.12).45

In 2009, Stratakis and co-workers reported the first biomimetic synthesis of acremine G (2.27),46_{in support of the proposed biosynthesis. Prenylated benzoquinone 2.9 was}

prepared from protected hydroquinone 2.39 in a five-step protocol and was able to undergo a Diels-Alder cycloaddition with unstable diene 2.40 yielding 2.41. Double deprotection of the silyl ethers gave rise to hydroquinone 2.42 which upon standing on air formed acremine G (2.27), possibly by a radical pathway (Scheme 2.6). Mehta showed one year later that the reaction proceeds even at room temperature on silica

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SCHEME 2.6: Stratakis‘ biomimetic synthesis of acremine G (2.27).46 support.47

In 2003, Porco and co-workers described the biomimetic synthesis of the dimeric natural product panepophenanthrin (2.42), consisting of two units of enone 2.43, which is structurally related to the acremine family (scheme 2.7).48_{Phenol 2.44 is converted into}

enone 2.45 by oxidation, transketalization and epoxidation. Further manipulations then elaborated tertiary alcohol 2.46 as a key intermediate. Global deprotection using aqueous HF uncaged enone 2.43 which spontaneously underwent a Diels-Alder cycloaddition to form pentacycle 2.47, which could not be isolated. 2.47 underwent further intramolecular hemiketal formation and therefore stabilizes the otherwise thermodynamically less stable dimeric structure of panepophenanthrin (2.42).

SCHEME 2.7: Porco’s biomimetic synthesis of panepophenanthrin (2.42).48

Danishefsky reported a synthesis of dysidiolide (2.48) involving a Gassman Diels-Alder reaction to form the core of the natural product.49_{Dioxolane 2.49 was prepared by a}

three-component coupling employing alkyne 2.50, dimethyl cuprate and iodide 2.51. Reduction followed by protection of the primary alcohol gave dioxolane 2.52 as the key

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step precursor. 2.52 underwent a cationic Diels-Alder reaction with diene 2.53 to form the complete core of dysidiolide. Further manipulations installed the furan moiety that, upon oxidation using singlet oxygen, furnished 2.48 (Scheme 2.8).

SCHEME 2.8: Danishefsky’s synthesis of dysidiolide (2.48).49

2.1.3 Vinyl quinone Diels-Alder reactions in total synthesis

The first Diels-Alder reaction involving a vinyl quinone as a diene was reported in 1998 by Irngartinger50_{(scheme 2.9). When examining the chemistry of aryl-substituted}

vinylquinones, an unexpected dimerization was observed that proceeded via an inverse electron demanding Diels-Alder reaction. This methodology was extended in 1999 by Noland,51_{when he showed that electron-deficient vinylquinones can undergo a}

Diels-Alder reaction with electron-rich alkenes. The resulting isoquinonemethide can be trapped with a nucleophile or, in many cases, tautomerize to the corresponding quinone.

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SCHEME 2.9: Initial observations of VQDA reactions.50-51

The first application of a VQDA reaction in total synthesis was reported by Trauner and co-workers in the synthesis of halenaquinone (2.61) in 2008.52_{This report also marks}

the first example of an intramolecular VQDA reaction (scheme 2.10). 2.61 aroused much interest in the synthetic community due to its interesting polycyclic framework and versatile biological profile. Alcohol 2.62 was derived enantioselectively from 2,3-diiodofurane (2.63) and aldehyde 2.64 in a three-step protocol. Further manipulations yielded aldehyde 2.65 which could undergo an intramolecular Heck reaction to give alkene 2.66 as a single diastereoisomer. Nucleophilic addition followed by oxidation then gave rise to protected hydroquinone 2.67. The key-step precursor was obtained by oxidation using AgO and HNO3 and smoothly underwent the desired VQDA reaction

under elevated pressure. Even though the reaction proceeded at room temperature, the yield could be significantly improved using high pressure conditions. The initial resulting isoquinonemethide could not be isolated but underwent fast tautomerization to the corresponding hydroquinone 2.68. Halenaquinone (2.61) was obtained in good yields after further oxidation.

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SCHEME 2.10: Trauner’s synthesis of halenaquinone (2.61).52

In 2009, Trauner expanded this methodology to the biomimetic total synthesis of (–)-pycnanthuquinone C (2.71)53_{anticipating that this reaction plays a significant role in the}

biosynthesis of secondary metabolites. Heck coupling of hydroquinone 2.72 with (–)-linalool (2.73) yielded styrene 2.74 that could be further oxidized to the key vinyl quinone

2.75 using MnO2. When 2.75 was slightly heated in a mixture of PhMe and H2O, the

VQDA reaction proceeded smoothly to give isoquinonemethide 2.76 which was directly intercepted by water in a nonstereospecific manner. The resulting hydroquinone 2.77 could not be isolated as it was spontaneously oxidized upon

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exposure to air to give (–)-pcynanthuquinone C (2.71) and its epimer 2.78, which Trauner predicted to be another natural product (Scheme 2.11).

In the same publication, Trauner proposed that a VQDA reaction is a plausible pathway for the biosynthesis of rosinone B (2.79) as well, and in 2010, Zhang and co-workers reported the biomimetic synthesis of 2.79.54_{Enone 2.80 was obtained by a three-step}

protocol from 2.81 and yielded acetate 2.82 after further manipulations. Hydrolysis of the acetate proceeded smoothly under basic conditions and gave protected hydroquinone

2.83 in good yields. 1,3-Allylic isomerization towards the thermodynamically more stable

styrene followed by MOM deprotection and oxidation afforded vinyl quinone 2.84 in excellent yield. Heating a solution of 2.84 in PhMe gave rise to isoquinonemethide 2.85 which was directly treated with p-TsOH in a mixture of MeOH and H2O. SN2’

displacement of methoxide afforded quinone 2.86, that could undergo a second intramolecular SN2’ displacement to give rosinone B (2.79) (Scheme 2.12).

SCHEME 2.12: Zhang’s synthesis of rosinone B (2.79).54

In 2013, Trauner and co-workers reported the total synthesis of (–)-isoglaziavianol (2.87),55_{marking the first example of trapping of the in situ generated isoquinonemethide}

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by an intramolecularly provided nucleophile (Scheme 1.13). Enone 2.88 is derived from primary alcohol 2.89 by a two-step protocol. Acetate cleavage and oxidation afforded vinylquinone 2.90 that spontaneously underwent the VQDA reaction at room temperature to form the isoquinonemethide 2.91. Intramolecular attack of the primary hydroxyl group and tautomerization gave hydroquinone 2.92 that was oxidized upon work-up, affording quinone 2.93. (–)-Isoglaziavianol (2.87) was obtained in good yield after removal of the PMB protecting group and reduction of the quinone moiety.

SCHEME 2.13: Trauner’s synthesis of isoglaziovianol (2.87).55

2.2 Project outline

Acremines are a rich class of meroterpenoid natural products mainly isolated from the Acremonium fungal species A. byssoides and A. persicinum.6-7, 39-44_{They are mainly}

simple meroterpenoids comprising an isopentenyl unit linked to a C7 tetraketide ring and

can be defined as C12 merohemiterpenoids. Acremines A–F and H–T are monomers

containing a single C12 unit, acremine G (2.7) was the first dimeric derivative consisting

of two C12 units. It is presumably generated from acremine A (2.1) and B (2.2) by a

Diels-Alder reaction and subsequent oxidative coupling. Bisacremines E (2.25) and F (2.26) consist of an unusual tetracyclic core that is highly oxidized and features ten stereogenic centers. Bisacremine G (2.27), bearing six stereogenic centers, consists of a heteropentacyclic core and exhibits inhibitory effects on the production of TNF-, IL-6, and nitric oxide in LPS-stimulated macrophages. 2.25 and 2.26 are supposed to be

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biosynthetically derived from two acremine F (2.6) units which undergo a formal [4+2] cycloaddition to install the full carbon core and further condensation to complete the southern tetrahydrofuran ring. 2.27 is proposed to be derived from 2.25 through further oxidation and cyclization. Even though this biosynthesis seems to be appealing, it remained unclear how this [4+2] cycloaddition would occur in nature. As both the endo and the exo transition state would give natural products, we assumed that this reaction would not occur in an enzymatic pocket but could proceed spontaneously in solution. In order to prove these hypotheses, we wanted to synthesize 2.25 and 2.26 starting from

2.6 (Figure 2.4).

FIGURE 2.4: Proposed biosynthetic pathway of bisacremine E (2.25).

In addition, we thought we might as well be able to get access to the more bioactive natural product bisacremine G (2.27). We envisioned that a vinyl quinone Diels-Alder reaction could give fast excess to the core of the natural product (Figure 2.5) and therefore expand the scope of this powerful methodology.

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2.3 Results and discussion

2.3.1 The cationic cascade

Bisacremine E (2.25) and its isomer bisacremine F (2.26) are supposed to be derived from acremine F (2.6) by a formal [4+2] cycloaddition followed by condensation to implement the final tetrahydrofuran ring.7_{Though the electronic properties would not be}

matching for a classical Diels-Alder reaction involving an electron-poor dienophile and an electron-rich diene, we thought that this reaction might be ionic in nature. We envisioned to synthesize 2.25 through a cationic cascade, depicted in Scheme 2.14.

SCHEME 2.14: Proposed biomimetic cascade to form bisacremine E (2.25).

Upon treatment with acid the tertiary allylic alcohol 2.6 was supposed to expel water forming a stabilized allylic cation 2.96 which could be trapped by the trisubstituted double bond of a second molecule of 2.6, again forming a stabilized cation 2.97. 2.97 should then be intercepted by the neighbouring trisubstituted double bond, forming the six-membered ring of 2.98. Trapping of the tertiary cation by the alcohol should then yield the natural product. Depending on whether the initial attack would come from the top or the bottom face, either 2.25 or 2.26 should be formed.

Total synthesis of marine and terrestrial natural products and development of novel photochromic ligands for ion channels, G-protein coupled receptors and cytoskeletal proteins

der Fakultät für Chemie und Pharmazie

der Ludwig-Maximilians-Universität München